Pharmacological and toxicological processes occur across a wide range of spatial and temporal scales and include multiple organ systems. A Systems Biology in silico toxicological model must include submodels that cover the multiple scales and the multiple tissues relevant to human medicine and toxicology. We will develop a liver centered mechanism based multiscale in silico simulation framework for xenobiotic toxicity and metabolism that incorporates four key biological scales: Population genetic and exposure variation scale Physiologically Based Pharmacokinetic (PBPK) whole body scale Tissue level and multicellular scale Subcellular signaling and metabolic pathways scales The multiscale in silico simulation will be centered on the liver, a critical organ in many toxicological, pharmacological, normal and disease processes. For our initial simulations of toxic challenge to the liver we will build a mechanism based in silico simulation of Acetaminophen (APAP) toxicity. APAP is a widely used over-the-counter pain reliever and fever reducer. An acute overdose of APAP is a leading cause of liver failure in the western world. APAP overdose leads to centrilobular liver necrosis that can progress to liver failure and in some cases patient death. Our multiscale in silico simulation will link existing open source modeling tools for the various spatiotemporal scales into an aggregate in silico model. This approach allows us to leverage existing tools, modeling modalities and models at the individual biological scales. Furthermore, this approach facilitates swapping models at individual scales without extensive modification of the sub-models at the other scales and allows us to leverage existing model development tools and resources. The complete multiscale in silico model will provide a mechanism based framework that incorporates effects at the various scales and will also provide a framework to predict changes in clinically used serum markers of liver function and failure. Our in silico simulation will be calibrated using microscopic imaging in the liver of a living mouse, mouse liver immune-histology, along with standard histology and serology in animal studies of APAP toxicity. The proposed in silico model is a first step in toxicity prediction 1. 2. 3. 4. simulatio that ultimately will lead to improved techniques for prediction toxicity of therapeutic agents and environmental toxins while simultaneously reducing the need for animal toxicity studies.